Group-III nitride (often referred to as III-nitride, or III-N) compounds, such as gallium nitride (GaN) and its related alloys, have been under intense research in recent years due to their promising applications in electronic and optoelectronic devices. Particular examples of potential optoelectronic devices include blue light emitting diodes and laser diodes, and ultra-violet (UV) photo-detectors. The large bandgap and high electron saturation velocity of the III-nitride compounds also make them excellent candidates for applications in high temperature and high-speed power electronics.
Due to the high equilibrium pressure of nitrogen at typical growth temperatures, it is extremely difficult to obtain GaN bulk crystals. Owing to the lack of feasible bulk growth methods, GaN is commonly deposited epitaxially on substrates such as SiC and sapphire (Al2O3).
The existing GaN formation process, however, suffers from drawbacks. The conventional GaN films formed from a substrate are often Ga-faced, meaning that after the deposition of a GaN layer is finished, there is a gallium layer, although typically very thin, on top of the GaN layer. In the patterning of the GaN layer, this gallium layer must be patterned first. However, due to the significant difference in the etching characteristics between the gallium layer and the GaN layer, the etchant commonly used for patterning GaN layers, for example, KOH solution, may not be able to attack the gallium layer efficiently. Therefore, instead of using the wet etch that has a greater throughput, dry etch has to be used for patterning the GaN layer, resulting in reduced throughput.
One existing solution to solve the above-discussed problem is to perform a nitridation on the substrate, for example, a silicon substrate, before forming a buffer/nucleation layer (on which the GaN layer is formed). However, this method results in a silicon nitride layer being formed on the silicon substrate. Due to the amorphous structure of the silicon nitride layer, the crystalline structure of the subsequently formed GaN layer is adversely affected. Further, silicon nitride is not conductive, and hence bottom electrodes cannot be formed on the backside of the substrate.
An additional problem is that the GaN layer often has a relatively great concentration of nitrogen vacancies. This limits the carrier concentrations in p-type GaN films. Accordingly, a new method for forming GaN layers having improved reaction to etching, improved process flexibility, and improved carrier concentrations is needed.